why UniqueConstraint doesn't work in flask_sqlalchemy

I want an alternative of Django's unique_together in flask, seems UniqueConstraint is what I'm looking for, but doesn't work for me.

here is the example:

import os
from flask import Flask
from flask_script import Manager, Shell
from flask_sqlalchemy import SQLAlchemy

basedir = os.path.abspath(os.path.dirname(__file__))

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] =\
    'sqlite:///' + os.path.join(basedir, 'data.sqlite')
app.config['SQLALCHEMY_COMMIT_ON_TEARDOWN'] = True
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

manager = Manager(app)
db = SQLAlchemy(app)

class User(db.Model):
    __tablename__ = 'users'
    __table_args__ = tuple(db.UniqueConstraint('name', 'address'))
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), nullable=False)
    address = db.Column(db.String(64), nullable=False)

    def __repr__(self):
        return '<User (%s, %s)>' % (self.name, self.address)


def make_shell_context():
    return dict(app=app, db=db, user=User)
manager.add_command("shell", Shell(make_context=make_shell_context))


if __name__ == '__main__':
    manager.run()

Test it:

$ python test.py shell

In [1]: db.create_all()

In [2]: u1=user(name='a', address='x'); u2=user(name='a', address='x'); 
db.session.add(u1); db.session.add(u2); db.session.commit()

In [3]: user.query.all()
Out[3]: [<User (a, x)>, <User (a, x)>]

I also tried with:

class User(db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), nullable=False)
    address = db.Column(db.String(64), nullable=False)
    db.UniqueConstraint('name', 'address')

not work either, what's wrong with it?